This invention relates to a method for recovering high purity chlorine in high yield from a chlorine gaseous mixture containing carbon dioxide as one component thereof. More particularly, this invention relates to a method for recovering chlorine as substantially pure liquid chlorine by a rectification operation of total-reflux type for a cooling process and by controlling the temperature of the liquid chlorine stored in the bottom of a rectification column to release carbon dioxide dissolved in the liquid chlorine.
Various methods for recovering chlorine from chlorine-containing gases have been proposed with respect to the chlorine liquefying process in brine electrolysis for the production of chlorine and to further treatment of its waste gas, examples of these proposals being as follows.
1. U.S. Pat. No. 2,540,905 describes a method for recovering chlorine substantially free from carbon dioxide from the waste gas of brine electrolysis, which contains 10 to 25% of chlorine as well as CO.sub.2, CO, H.sub.2, N.sub.2 and O.sub.2, by absorbing chlorine with a suitable chlorinated hydrocarbon solvent, such as heptachloropropane, and simultaneously desorbing the absorbed carbon dioxide at different temperatures.
2. British Pat. No. 798,576 also describes a method for stripping chlorine by controlling the CO.sub.2 partial pressure in the chlorine liquefying waste gas of brine electrolysis below 1.5 atm under compression to absorb chlorine in water at 10.degree. to 100.degree.C and then heating the resulting solution under reduced pressure.
3. H. Hagemann (Chemie Ing. Tech. 39, 744-747 (1967)) describes a method for achieving the chlorine liquefaction in high yield by blowing compressed cell gas (anode chamber gas) of brine electrolysis into liquid chlorine to purge carbon dioxide dissolved in the liquid chlorine and recycling the purged carbon dioxide with the cell gas into the liquefying step to avoid the explosion accompanying stored hydrogen when liquefying the cell gas of brine electrolysis. Concerning the methods of the above references (1) and (2), however, they are not economical, because the absorption or adsorption techniques, as tools for separating chlorine from the chlorine containing gaseous mixtures, deal essentially with dilute chlorine, whereas, in the industrial treatment of the chlorine-containing gas, in which the chlorine concentration amounts to 50% or more, it is necessary to use a large quantity of absorbents or adsorbents and, moreover, to set up processes for desorbing and recovering of chlorine from these absorbents and adsorbents. As for the method of the above reference (3), the quantity of carbon dioxide contained in the cell gas of an ordinary brine electrolysis ranges only from about 0.5 to 1.5% by volume, so that it does not cause trouble in practical use, even if all of the carbon dioxide dissolves in the liquid chlorine. Further, when the carbon dioxide concentration in the cell gas is, say, 0.6%, and if it is enriched by the addition of carbon dioxide purged from liquid chlorine, its concentration in the cell gas increases to only about 1%, whereby there is no need to control the residual carbon dioxide in the liquid chlorine.